The present invention relates to a method and apparatus for a variable aperture ultrasonic scanner imaging system using separate transmit and receive multiplexers for independent placement of the transmit and receive apertures within the transducer array.
The number of processing channels in a commercial ultrasound imaging system is limited by the necessity of portability and the convenience of using standard electrical power outlets. In order to achieve higher lateral image resolution or better elevational resolution, transducer arrays today are manufactured with more elements than available processing channels on the imaging system. To access elements of such transducer arrays, multiplexers are used to connect a subset of the array elements to the available processing channels. One advantageous use of multiplexing is the creation of synthetic apertures, in which multiple firings and multiplexer selection of adjacent subarrays of the transducer array can be utilized to create the effect of beamforming over an effective aperture that covers the spatial extent of the combined subarrays. Such an increased aperture can result in an increased spatial resolution image. Thus, the number of elements in an array can be increased without a corresponding increase in the number of transmit or receive processing channels, but at the expense of decreasing the frame rate due to the multiple firings needed to create a synthetic aperture.
Prior art ultrasound scanners typically used transducer arrays with a number of elements that matched the number of processing channels used in the transmit and receive beamformers. When a transducer array was used for both transmission and reception, a transmit/receive (T/R) electronic switch was simply used to switch after firing from the transmitters to the receivers in order to start the receive beamforming as the echoes returned. A prior art system is illustrated in FIG. 3a. When larger transducer arrays with more elements than processing channels were fabricated, the original scanner designs were simply modified by the placement of a bidirectional multiplexer 400 in the signal path between the transducer connector and the T/R switch 300. A beam could then be synthesized using signals from the whole transducer aperture by a process of multiple firings and multiplexer repositioning of either the transmit aperture, receive aperture, or both before each firing.